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This can happen because the field property for [num1] called "Treat blank as zero in calculations" is checked. This is checked by default, so you can do things like add and subtract lists of values which contain some blanks.
When this is checked, a blank field looks to formulas just like a zero, which is not the same as null. For the Nz() formula to work correctly, uncheck this box. Then blank fields will then be seen as null.